When breath alcohol measuring instruments are used for forensic purposes, it is particularly important to establish that each measurement is accurate. The U.S. Department of Transportation (DOT), for example, requires 95% accuracy for a breath alcohol instrument which is used to establish the blood alcohol level of highway drivers.
In order to establish the accuracy of an instrument, its calibration is checked periodically and a notation is made that its calibration remains within legal tolerances. Ideally, the calibration check is made every time a suspect's breath is checked and its adherence to calibration is recorded automatically.
There has been long recognition that the reading of a standard is subject to several variables. The expected value of simulator (wet gas) must be at 34.degree. C. to supply a proper sample, while dry gas standards must be adjusted for barometric pressure. Calibration of such instruments with dry standards has been proposed by taking into account this variable. Such approaches recalibrate the instrument each time a standard is run. The present invention is not concerned with recalibrating the instrument, but rather with checking that the instrument is calibrated accurately.
The use of dry pressurized gas calibration standards for breath alcohol instruments has long been known. Hutson, U.S. Pat. No. 3,847,551, discloses using such a standard to overcome many of the complications inherent in calibrating and checking the calibration of a breath alcohol instrument. It has been found, however, that changes in atmospheric pressure (barometric pressure) produce a change in the reading given with the dry pressurized Standard, even when the instrument is properly calibrated and therefore gives accurate measurements when measuring breath alcohol. Therefore, changes in barometric pressure can make an instrument appear to be out of calibration when it in fact is properly calibrated, or conversely can make an instrument appear to be in calibration when it is not. Changes in atmospheric pressure may be caused not only by changes in elevation, but by changes in pressure in weather. Although the pressure changes caused by elevational differences are constant with time, weather changes can produce apparent, spurious, changes in calibration amounting to as much as five percent.